INTERNET-DRAFT R. Housley
Intended Status: Proposed Standard Vigil Security
Expires: 27 February 2014 26 August 2013
Use of the Hash-based Merkle Tree Signature (MTS) Algorithm
in the Cryptographic Message Syntax (CMS)
Abstract
This document specifies the conventions for using the Merkle Tree
Signatures (MTS) digital signature algorithm with the Cryptographic
Message Syntax (CMS). The MTS algorithm is one form of hash-based
digital signature.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. MTS Digital Signature Algorithm . . . . . . . . . . . . . 3
1.2. LDWM One-time Signature Algorithm . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. Algorithm Identifiers and Parameters . . . . . . . . . . . . . 5
3. Signed-data Conventions . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4.1. Implementation Security Considerations . . . . . . . . . . 6
4.2. Algorithm Security Considerations . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix: ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
This document specifies the conventions for using the for using the
Merkle Tree Signatures (MTS) digital signature algorithm with the
Cryptographic Message Syntax (CMS) [CMS] signed-data content type.
The MTS algorithm is one form of hash-based digital signature that
can only be used for a specific number of signatures. The MTS
algorithm is described in [HASHSIG]. The MTS algorithm uses small
private and public keys, and it has low computational cost; however,
the signatures are quite large.
CMS values are generated using ASN.1 [ASN1-02], using the Basic
Encoding Rules (BER) and the Distinguished Encoding Rules (DER).
1.1. MTS Digital Signature Algorithm
Merkle Tree Signatures (MTS) are a method for signing a large but
fixed number of messages. An MTS system uses two cryptographic
components: a one-time signature method and a collision-resistant
hash function. Each MTS public/private key pair is associated with a
k-way tree with each node containing an n-byte value. Each leaf of
the tree contains the value of the public key of an Lamport, Diffie,
Winternitz, and Merkle (LDWM) public/private key pair [HASHSIG]. The
LDWM algorithm requires a robust one-way function to underpin the
signature generation and verification. The algorithms in this
document all make use of the SHA-256 [SHS] one-way hash function,
which produces a 32 byte result.
The value at the root of the tree is the MTS public key. Each
interior node is computed by applying the hash function to the
concatenation of the values of its children nodes. Once again, the
algorithms in this document all make use of the SHA-256 [SHS] one-way
hash function.
An MTS signature consists of an LDWM signature, a node number that
identifies the leaf node associated with the signature, and an array
of values associated with the path through the tree from the LDWM
signature leaf to the root. The array of values contains contains
the siblings of the nodes on the path from the leaf to the root but
does not contain the nodes on the path itself. The array for a tree
with branching number k and height h will have (k-1)*h values. The
first (k-1) values are the siblings of the leaf, the next (k-1)
values are the siblings of the parent of the leaf, and so on.
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Four tree sizes are specified in [HASHSIG]:
MTS_SHA256_K2_H20:
o k = 2 (2 child nodes for each interior node),
o h = 20 (20 levels in the tree),
o n = 32 (32 bytes associated with each node), and
o mts_algorithm_type = 0x00000001.
MTS_SHA256_K4_H10:
o k = 4 (4 child nodes for each interior node),
o h = 10 (10 levels in the tree),
o n = 32 (32 bytes associated with each node), and
o mts_algorithm_type = 0x00000002.
MTS_SHA256_K8_H7:
o n = 8 (8 child nodes for each interior node),
o h = 7 (7 levels in the tree), and
o n = 32 (32 bytes associated with each node), and
o mts_algorithm_type = 0x00000003.
MTS_SHA256_K16_H5:
o k = 16 (16 child nodes for each interior node),
o h = 5 (5 levels in the tree),
o n = 32 (32 bytes associated with each node), and
o mts_algorithm_type = 0x00000004.
There are k^h leaves in the tree.
1.2. LDWM One-time Signature Algorithm
Merkle Tree Signatures (MTS) depend on a LDWM one-time signature
method. The four variants described in [HASHSIG] depend on SHA-256
[SHS] and SHA-256-20, which is the same as SHA-256, except that the
hash result is truncated to 20 bytes.
Four LDWN one-time signature algorithms are defined in [HASHSIG]:
LDWM_SHA256_M20_W1:
o ldwm_algorithm_type = 0x00000001; and
o the signature value is the 4-byte ldwm_algorithm_type
followed by 265 20-byte values.
LDWM_SHA256_M20_W2:
o ldwm_algorithm_type = 0x00000002; and
o the signature value is the 4-byte ldwm_algorithm_type
followed by 133 20-byte values.
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LDWM_SHA256_M20_W4:
o ldwm_algorithm_type = 0x00000003; and
o the signature value is the 4-byte ldwm_algorithm_type
followed by 67 20-byte values.
LDWM_SHA256_M20_W8:
o ldwm_algorithm_type = 0x00000004; and
o the signature value is the 4-byte ldwm_algorithm_type
followed by 32 20-byte values.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [KEYWORDS].
2. Algorithm Identifiers and Parameters
The algorithm identifier for an MTS signature is id-alg-mts-hashsig:
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-alg OBJECT IDENTIFIER ::= { id-smime 3 }
id-alg-mts-hashsig OBJECT IDENTIFIER ::= { id-alg 17 }
When the id-alg-mts-hashsig algorithm identifier is used for a
signature, the AlgorithmIdentifier parameters field MUST be absent.
The first 4 bytes of the signature value contains the
mts_algorithm_type as defined in Section 4.5 of [HASHSIG]. For
convenience, these values are repeated in above in Section 1.1 of
this document. This value tells how to parse the remaining parts of
the signature value, which is composed of an LDWM signature value, a
4-byte signature leaf number, and the MTS path.
The first 4 bytes of the LDWM signature value contains the
ldwm_algorithm_type as defined in Section 3.10 of [HASHSIG]. For
convenience, these values are repeated in above in Section 1.2 of
this document.
The signature format is designed for easy parsing. Each format
starts with a 4-byte enumeration value that indicates all of the
details of the signature algorithm, indirectly providing all of the
information that is needed to parse the value during signature
validation.
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3. Signed-data Conventions
digestAlgorithms SHOULD contain the one-way hash function used to
compute the message digest on the eContent value. Since the hash-
based signature algorithms all depend on SHA-256, it is strongly
RECOMMENDED that SHA-256 also be used to compute the message digest
on the content.
Further, the same one-way hash function SHOULD be used to compute the
message digest on both the eContent and the signedAttributes value if
signedAttributes exist. Again, since the hash-based signature
algorithms all depend on SHA-256, it is strongly RECOMMENDED that
SHA-256 be used.
signatureAlgorithm MUST contain id-alg-mts-hashsig. The algorithm
parameters field MUST be absent.
signature contains the single value resulting from the signing
operation.
4. Security Considerations
4.1. Implementation Security Considerations
Implementations must protect the private keys. Compromise of the
private keys may result in the ability to forge signatures. Further,
a LDWM private key MUST be used only one time, and the LDWM private
key MUST NOT be used for any other purpose.
The generation of private keys relies on random numbers. The use of
inadequate pseudo-random number generators (PRNGs) to generate these
values can result in little or no security. An attacker may find it
much easier to reproduce the PRNG environment that produced the keys,
searching the resulting small set of possibilities, rather than brute
force searching the whole key space. The generation of quality
random numbers is difficult. RFC 4086 [RANDOM] offers important
guidance in this area.
When computing signatures, the same hash function SHOULD be used for
all operations. This reduces the number of failure points in the
signature process.
4.2. Algorithm Security Considerations
At Black Hat USA 2013, some researchers gave a presentation on the
current sate of public key cryptography. They said: "Current
cryptosystems depend on discrete logarithm and factoring which has
seen some major new developments in the past 6 months" [BH2013].
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They encouraged preparation for a day when RSA and DSA cannot be
depended upon.
A post-quantum cryptosystem is a system that is secure against
quantum computers that have more than a trivial number of quantum
bits. It is open to conjecture whether it is feasible to build such
a machine. RSA, DSA, and ECDSA are not post-quantum secure.
The LDWM one-time signature and MTS system do not depend on discrete
logarithm or factoring, and these algorithms are considered to be
post-quantum secure.
Today, RSA is often used to digitally sign software updates. This
means that the distribution of software updates could be compromised
if a significant advance is made in factoring or a quantum computer
is invented. The use of MTS signatures to protect software update
distribution, perhaps using the format described in [FWPROT], will
allow the deployment of software that implements new cryptosystems.
5. IANA Considerations
{{ RFC Editor: Please remove this section prior to publication. }}
This document has no actions for IANA.
6. References
6.1. Normative References
[ASN1-02] ITU-T, "ITU-T Recommendation X.680, X.681, X.682, and
X.683", ITU-T X.680, X.681, X.682, and X.683, 2002.
[CMS] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[HASHSIG] McGrew, D., and M. Curcio, "Hash-Based Signatures", Work
in progress.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SHS] National Institute of Standards and Technology (NIST),
FIPS Publication 180-3: Secure Hash Standard, October
2008.
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6.2. Informative References
[BH2013] Ptacek, T., T. Ritter, J. Samuel, and A. Stamos, "The
Factoring Dead: Preparing for the Cryptopocalypse", August
2013.
[https://media.blackhat.com/us-13/us-13-Stamos-The-
Factoring-Dead.pdf]
[CMSASN1] Hoffman, P. and J. Schaad, "New ASN.1 Modules for
Cryptographic Message Syntax (CMS) and S/MIME", RFC 5911,
June 2010.
[FWPROT] Housley, R., "Using Cryptographic Message Syntax (CMS) to
Protect Firmware Packages", RFC 4108, August 2005.
[PKIXASN1] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
June 2010.
[PQC] Bernstein, D., "Introduction to post-quantum
cryptography", 2009.
[http://www.pqcrypto.org/www.springer.com/cda/content/
document/cda_downloaddocument/9783540887010-c1.pdf]
[RANDOM] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
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Appendix: ASN.1 Module
MTS-HashSig-2013
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
id-smime(16) id-mod(0) id-mod-mts-hashsig-2013(64) }
DEFINITIONS EXPLICIT TAGS ::= BEGIN
EXPORTS ALL;
IMPORTS
SIGNATURE-ALGORITHM PUBLIC-KEY
FROM AlgorithmInformation-2009 -- RFC 5911 [CMSASN1]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) }
mda-sha256
FROM PKIX1-PSS-OAEP-Algorithms-2009 -- RFC 5912 [PKIXASN1]
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-rsa-pkalgs-02(54) } ;
--
-- Object Identifiers
--
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs9(9) 16 }
id-alg OBJECT IDENTIFIER ::= { id-smime 3 }
id-alg-mts-hashsig OBJECT IDENTIFIER ::= { id-alg 17 }
--
-- Signature Algorithm and Public Key
--
sa-MTS-HashSig SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-alg-mts-hashsig
HASHES { mda-sha256, ... }
PUBLIC-KEYS { pk-MTS-HashSig } }
pk-MTS-HashSig PUBLIC-KEY ::= {
IDENTIFIER id-alg-mts-hashsig
KEY MTS-HashSig-PublicKey }
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MTS-HashSig-PublicKey ::= OCTET STRING
HashSignatureAlgs SIGNATURE-ALGORITHM ::= {
sa-MTS-HashSig, ... }
END
Author's Address
Russ Housley
Vigil Security, LLC
918 Spring Knoll Drive
Herndon, VA 20170
USA
EMail: housley@vigilsec.com
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